Particle sampling is typically accomplished by either "active" or "passive" techniques. When air samples containing airborne particles are pumped through and captured onto a filter, the sampling is active. When airborne particles are allowed to settle naturally onto a collection surface, the sampling is passive. A number of materials have been used as collection surfaces, often as collection "plates," for passive sampling. These materials include carbon-coated sheets of thin mica, polycarbonate filter material, quartz crystals and glass slides.
Unfortunately, the inert surfaces typically used for passive sampling of airborne particulates inherently results in some particle loss--prior to particle analysis--in one of two ways. First, due to the nature of the collection surface and depending upon the particle size and settling or impact velocity, a particle may bounce free of the collection material. Second, because the particles are not physically fixed or held to the collection surface, they are easily dislodged and lost from the collection surface between collection and particle evaluation--particularly because, as is discussed further below, evaluation is seldom if ever conducted at the sampling site itself.
The media used for passive collection should be suitable for both optical and electron microscopy examination. Although limited by resolution and magnification, optical microscopy provides a quick and overall view of the sample without damaging the specimen. Electron microscopy provides higher resolution and magnification and in addition provides the capability of determining the chemical composition of particles by energy dispersive x-ray spectroscopy. Two methods of electron microscopy, Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) are used. The collection media should also be suitable for application of other methods of particle characterization such as x-ray diffraction (XRD) and atomic absorption (AA).
Particle deposition plates preferably should be of a type to allow direct and indirect microscopic analysis. If it is relevant to study the relative orientation and the distribution of the particles as deposited, a direct preparation method is used. With direct preparation techniques, the sample is examined in the as-received condition, i.e., directly on the collection plate. Indirect preparation for electron microscopy involves transferring the particles from the collection plate into a liquid and redepositing them onto a filter membrane to obtain a uniform distribution. The filter membrane is then prepared for electron microscopic examination, during which accurate particle concentration assessment may be made.
Conventional attempted solutions for the problem of particle loss from collection surfaces have included the use of coatings of grease or adhesive on the collection surface, and these techniques have resulted in at least some limited success. With grease or adhesives, however, the coated collection surface creates problems of sample preparation which are as serious if not more serious than the original particle loss. Without controlled tack--which is completely impossible with some adhesives and virtually all greases--the resultant sample fouling presents a worse problem than the collection problem the greases and/or adhesives were intended to solve.
The passive sampling technology therefore needs an improved method, and means for effecting the method, of collecting airborne particulates with enhanced particle capture and retention and for doing so without interfering with SEM, TEM or other methods of examination.